add PROCESSING_TIMEOUT upload error code

[matt-codecov]

a significant number of "unknown errors" that we report to users are timing out during processing. this PR creates an error code for it, and we can later choose to display different text in gazebo for that error code

note: worker will attempt to set this error code and save everything when it gets the "soft timeout" signal. but there is no guarantee it won't be forcibly killed before it can save everything. there will still be some unknown uploads from that

sibling PR in gazebo: codecov/gazebo#3522

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.81%. Comparing base (511b553) to head (10e4737).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #431      +/-   ##
==========================================
- Coverage   90.56%   89.81%   -0.75%     
==========================================
  Files         390      324      -66     
  Lines       11768     9379    -2389     
  Branches     1974     1677     -297     
==========================================
- Hits        10658     8424    -2234     
+ Misses       1026      891     -135     
+ Partials       84       64      -20     

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